Filemaker is very good about different install locations for different versions of the program. If you accept the defaults while you are installing the programs, they will automatically be installed to different folders.
As far a sharing DBs, you will only be able to share from one Filemaker program at a time. The first program you open will take ownership of the port (unless web sharing is disabled). The second program will flash a warning message telling you that the port is not available and that it cannot share files.
You shouldn't have any major problems with a "dual install" like this. I have 5.5 and 10 on my machine due to recently completing just such a conversion.
The main issue is to take care with your Windows settings that control which application will launch when you duoble click a database file.
Both FMP 10 and FMP 7 use the same file format and the same .fp7 file suffix so only one app will automatically launch when you open a file. To open the file with the other version, simply launch the application first, then choose open from the file menu.
As to sharing, you should open the file first on you host computer. Thus you can control which version hosts the file simply by taking care to select the correct version to open in in the first place. (You can probably just host the file in fmp 10 and fmp 7 users will be able to connect to the db without any trouble since the file format hasn't changed.)